when I activate the EWWW plugin all my WordPress' administration pages go blank and I can't do anything. I solved it deleting the plugin from the FTP, so I didn't find a solution to make EWWW Image Optimizer work just yet, any clue?

Do you use the W3TC plugin? I've heard reports of quirks with that plugin, maybe need to flush all the caches, not sure quite what the deal is with that.

Good place to look would be your error logs. Typically, the only reason for a blank page is some sort of syntax error that makes wordpress unable to load.
Generally though, the only way that happens is if the plugin was corrupted when you installed it. Try downloading it fresh again, preferably using the built-in WP plugin installer.